Design of Dual-Band Coupler with High Frequency Ratio and Strong Coupling Strength

In modern communication systems, coupled-line couplers are useful because they can provide output signals with various coupling power ratios and with 90 phase difference. The electrical length of the conventional coupled-line coupler is required to be quarter wavelength, which lets them only operate at a single band. The research of dual-band coupled-line couplers was seldom investigated. The control of the bandwidths was difficult, and the frequency ratio was limited to less than three because of the impedance limitation of microstrip fabrication. This two-year research project designs a series of novel dual-band coupled-line couplers. The task for the first year develops the dual-band stub-loaded coupled-line coupler with high frequency ratio. The structure of two coupled-line sections taped by stubs in between is proposed to achieve dual band operation. The dual center frequencies are controllable by tuning the electrical length of coupled lines and stubs, while the bandwidth is controlled by the stub impedance. The electrical length of the proposed couplers reduces as the frequency ratio increases. In our studies, the spurious responses will exist when frequency ratio ＞ 4, resulting in undesired output signals. Accordingly, this project will develop a novel strip-via EBG structure. By combining the EBG and the proposed coupler, a dual-band coupler with high frequency ratio and superior spurious suppression can be achieved. Conventionally, coupled-line couplers are difficult to realize 3 dB coupling strength. The reason is the high coupling always requires high ratio of even-mode impedance to odd-mode impedance, which results in unrealizable microstrip line width and spacing. Therefore, the mentioned stub-loaded coupled-line coupler is only useful for weak coupling applications. For providing strong coupling, the task for the second year continues the research of the first year to develop dual-band couplers with high frequency ratio and strong coupling strength. Four stubs and two uncoupled transmission lines are added to the coupled lines to construct a multi-coupled structure to enhance the coupling strength. The proposed structure is easy to realize 3 dB coupling at the cost of increasing the circuit size. Moreover, the bandwidth can be controlled and the even-mode impedance can be reduced, by the stub impedance, for easy fabrication. The spurious responses will also happen when frequency ratio is high. A dual-band coupler with high frequency ratio, strong coupling strength, and spurious suppression can be designed by combining the EBG structure and couplers. Finally, the monosymmetric structure of coupled-line coupler is proposed with the associated synthesis conditions to extend the 90 degree phase difference to 180 degree, which can achieve similar performance of dual-band rat-race coupler and balun.

Project ID:PB10408-5708
External Project ID:MOST104-2221-E182-007